1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer or a facsimile machine, and a method of controlling the image forming apparatus.
2. Description of the Related Art
Conventionally, in an image forming apparatus using electrophotography, a fixing device has been widely used which incorporates a fixing heat roller that causes a sheet having a toner image transferred thereon to pass therethrough for heating and pressing the sheet, to thereby fix the unfixed toner image onto the sheet.
FIG. 10 shows a control circuit for the conventional fixing device. In the control circuit, there are arranged a halogen heater 21, a thermistor 22, A/D converters 23 and 24, a control section 25, an energization pattern-generating section 26, a heater drive circuit 27, and so forth.
The halogen heater 21 generates heat by being supplied with electric power. Energization of the heater is controlled such that the resistance value of the thermistor 22, which is disposed as a temperature-detecting element in a state in contact with the surface of a fixing roller, not shown, becomes constant with respect to a reference value.
The A/D converter 23 converts a voltage VT obtained according to a voltage-dividing ratio between the resistance value of a resistance RY of the thermistor 22 and the resistance value of a resistance R1 into a digital value thereof. The A/D converter 24 converts a control target voltage Vref1 into a digital value thereof. The A/D converters 23 and 24 output digital values SG1 and SG2 to the control section 25, respectively.
The energization pattern-generating section 26 delivers a heater control signal SG3 to the heater drive circuit 27 based on a signal SG4 from the control section 25.
In response to an input signal from a sensor 28, the control section 25 controls the temperature of the fixing roller by controlling the heating of the halogen heater 21 based on the digital value SG1 of the voltage VT applied to the thermistor 22, using the signal SG2 as a digital value of the control target voltage Vref1 optimum for fixing a toner image.
FIG. 11 shows an electrophotographic image forming apparatus.
This image forming apparatus is comprised of a photosensitive drum 1101 as an electrostatic latent image bearing member, a semiconductor laser 1102 as a light source, and a rotating polygon mirror 1103. A laser beam 1104 generated by the semiconductor laser 1102 scans the surface of the photosensitive drum 1101 via the rotating polygon mirror 1103 to thereby form an electrostatic latent image on the photosensitive drum 1101.
It should be noted that before the electrostatic latent image is formed on the photosensitive drum 1101, the surface of the photosensitive drum 1101 is uniformly charged by an electrostatic charging roller 1105. The electrostatic latent image formed on the photosensitive drum 1101 is developed into a toner image by a developing device 1106. The toner image is transferred on a sheet conveyed thereto, by a transfer roller 1107.
On the other hand, sheets are accommodated in a sheet feed cassette 1108 in a stacked state, and are fed from the sheet feed cassette 1108 into a conveying path by a sheet feed roller 1109.
Each sheet fed into the conveying path has a leading end thereof brought into abutment with a registration roller pair 1110 to have skew thereof corrected. Writing of image data on the photosensitive drum 1101 is performed in synchronism with sheet conveyance timing. The sheet is conveyed to a transfer position where a toner image formed on the photosensitive drum 1101 is transferred onto the sheet via the transfer roller 1107.
Disposed on the upstream side of the registration roller pair 1110 is a registration sensor 311 for detecting whether or not a sheet exits.
When the sheet having the toner image transferred thereon passes through a fixing roller 1112 along the conveying path, the sheet is heated and pressed to have the unfixed toner image fixed thereon. Then, the sheet having passed through the fixing roller 1112 is discharged from the apparatus by a discharge roller pair 1113, and the discharged sheet is detected by a sheet discharge sensor 1114.
FIG. 12 shows a control system of the image forming apparatus shown in FIG. 11.
Referring to FIG. 12, a printer controller 1201 converts image code data sent from a host computer into printable bitmap data. Further, the printer controller 1201 designates a print mode for the image forming apparatus and instructs the start of printing.
An engine controller 1202 controls mechanical units of the image forming apparatus based on instructions from the printer controller 1201.
A sheet conveyance controller 1203 performs operations e.g. for driving or stopping component parts of a conveyance system based on instructions from the engine controller 1202. A high voltage controller 1204 outputs high voltages for electrostatic charging, development of an image, and transfer of the image to a sheet, based on instructions of the engine controller 1202.
An optical system controller 1205 performs operations e.g. for driving or stopping a scanner motor, not shown, and flickering of the laser based on instructions from the engine controller 1202. A sensor input section 1206 receives information input from sensors, such as the registration sensor 311 and the sheet discharge sensor 1114, and sends the same to the engine controller 1202. A fixing device temperature controller 1207 controls the temperature of the fixing roller based on instructions from the engine controller 1202.
Next, the temperature control of the fixing roller will be described with reference to FIGS. 13A and 13B.
First, when the power is turned on, the engine controller 1202 initializes the image forming apparatus (step S1301), and then starts temperature control for holding the fixing roller at a standby temperature for a non-printing condition (step S1302).
This temperature control is executed by the CPU taking in a voltage value detected by a thermo-electric element (e.g. thermistor) attached in contact with the fixing roller, via an A/D converter, not shown, in the engine controller 1202.
The above A/D converted value and an A/D converted value corresponding to the standby temperature are compared with each other (step S1303).
Then, when the temperature of the fixing roller is higher than the standby temperature, a fixing heater is turned off (step S1304), whereas when the temperature of the fixing roller is lower than the standby temperature, the fixing heater is turned on (step S1305). This ON/OFF operation is carried out until a print request is received from the printer controller 1201.
When the print request is received, there is executed a process for starting the scanner motor, conveyance motors, not shown, of the conveyance system, drive sections for driving high voltage sections, and so forth, and causing the temperature of the fixing roller to rise to a printing temperature (steps S1307 to S1310).
After that, until the printing operation is terminated (step S1314), the ON/OFF operation of the fixing heater is continued in which when the temperature of the fixing roller is higher than the printing temperature, the fixing heater is turned off (step S1312), whereas when the temperature of the fixing roller is lower than the printing temperature, the fixing heater is turned on (step S1313).
It should be noted that the standby temperature of the fixing roller is set to be lower than the printing temperature, and the difference therebetween is fixed to a difference which is small enough for causing the temperature of the fixing roller to rise to the printing temperature before a sheet reaches the fixing roller after receipt of the print request.
However, in the above-described conventional temperature control of the fixing roller, the surface temperature of the fixing roller is detected by the thermistor or the like in contact with the fixing roller, and hence the contact sometimes causes a scratch on the surface of the fixing roller. In such a case, the scratch is transferred onto an image formed on a sheet passing the fixing roller, which produces a defective image.
To solve this problem, there has been proposed a technique that uses a non-contact detection sensor disposed in the vicinity of the surface of the fixing roller in a manner spaced therefrom and a compensation sensor for detecting the ambient temperature of the detection sensor, and estimates the surface temperature of the fixing roller using a computing equation based on information from the sensors (see e.g. Japanese Patent Laid-Open Publication No. 2003-149981).
More specifically, the computing equation is given by the surface temperature (° C.) of the fixing roller=(a×compensation voltage−b)×detected voltage+(c×compensation voltage+d). In this equation, the compensation voltage and the detected voltage are digital values which are obtained by A/D conversion of voltages detected by the compensation sensor and the detection sensor, respectively. Further, a, b, c and d represent coefficients, values of which are different depending on the compensation temperature.
In storing the computing equation, a conversion table is divided into a plurality of sections according to respective ranges of compensation temperature such that coefficients of the computing equation are determined on a section-by-section basis. The computing equation is stored together with each section of the table in an associated one of predetermined storage areas. This means that there are a plurality of computing equations having coefficients with different values. A computing equation to be used is determined depending on a compensation temperature detected by the compensation sensor, and the surface temperature of the fixing roller is calculated using the determined computing equation.
In the above-described proposal, it is assumed that by using the computing equations thus determined, the calculation of powers of values of the compensation temperature and the detection temperature can be eliminated to improve calculation speed.
However, according to the above-described Japanese Patent Laid-Open Publication No. 2003-149981, it is necessary to divide the conversion table into a plurality of sections according to ranges of the compensation temperature, and store coefficients determined on a section-by-section basis in each of the sections, to thereby store a plurality of computing equations. This requires complicated circuit configuration and control operations.